The object of the invention is a frequency synthesizer with a voltage-controlled oscillator that can be tuned over its total frequency range in increments of a prescribed frequency range by means of a frequency-dictating component and with a reference oscillator that emits a reference signal of a strictly prescribed frequency that is supplied in the form of an input signal to a comparator, to which is simultaneously supplied another input signal of a frequency that is a function of the frequency of the output signal from the voltage-controlled oscillator, the second input signal generating a fine-tuning signal in accordance with the difference between the frequencies or between the phases of the first and second input signals and the fine-tuning signal supplied to the voltage-controlled oscillator to fine-tune the frequency of its output signal to the level defined by the reference signal.
Various embodiments of frequency synthesizers of this type are known. The reference oscillator is generally a quartz oscillator that supplies a reference signal having an extremely stable frequency.
The second input signal, which is supplied to the comparator and which has a frequency that is supposed to be a function of the frequency of the output signal, can usually be generated due to the presence of at least one other reference oscillator, which can for example be a spectrum generator with an output signal that contains discrete values of a prescribed frequency spectrum. The output signal from the second reference oscillator is mixed with the output signal from the voltage-controlled oscillator and the output signal from the mixer is supplied by way of an appropriate band-pass filter to the comparator, wherein a signal with a frequency that is either the sum of or the difference between the frequency of the output signal from the voltage-controlled oscillator and the frequency of a signal with one of the frequencies of the second reference oscillator, is compared to the output signal from the first reference oscillator. The accordingly generated channel grid of signals corresponds to the interval between the spectral lines of the second reference oscillator. If this interval is for example 1 MHz, the output signal from the voltage-controlled oscillator can be stabilized at frequencies 1 MHz apart. The voltage-controlled oscillator is usually fine-tuned by supplying the fine-tuning signal emitted by the comparator by way of a loop filter that suppresses control oscillations to a capacitive diode that controls the voltage-controlled oscillator. The frequency synthesizer can be tuned at the variable capacitor to approximately the desired frequency and will accordingly lock into one of the prescribed frequencies, becoming tuned to the precision of the reference frequency.
If the frequency grid generated in this way seems to be too coarse, a third oscillator can be employed. The third oscillator can be a local oscillator with a frequency in the vicinity of switchable quartz crystals and with an output signal that can be mixed in another mixer with the signal generated from the signals from the voltage-controlled oscillator and from the spectrum generator, creating a fine-frequency grid.
A frequency grid with intervals of 25 kHz can be created in this way in the frequency range of 40 to 50 MHz for example for the output signal from the voltage-controlled oscillator, with the adjusted frequencies having a precision of approximately 2 to 3 kHz.
One drawback of this known frequency synthesizer is its expensive and complicated design, involving several reference oscillators and a series of circuitry measures to suppress disruptive subsidiary frequencies. One advantage of the known embodiments is that the coarse tuning is carried out with a low-loss variable capacitor and only the fine tuning is carried out by means of a high-loss fine-tuning diode, which is noisy by its very nature.
Also known is a frequency synthesizer that avoids the high circuitry expenditure of the aforesaid embodiment by supplying the output signal from the voltage-controlled oscillator to a variable frequency divider with a dividing factor that ensures that its output signal will be in the frequency range of the reference signal. The fine-tuning signal obtained from the comparator is supplied in this embodiment as well to a capacitive diode for tuning the voltage-controlled oscillator. This embodiment lacks a variable capacitor for coarse tuning, and the output frequency is dictated strictly by setting the dividing factor at the variable frequency divider.
A frequency synthesizer of this type is inexpensive to construct, and its frequency constancy depends on only a single reference oscillator. Its essential drawback is that, since a capacitive diode is employed for all the tuning, a considerable amount of noise must be put up with.